Some conventional main transistors can scale output power on demand by selectively controlling different active regions of the transistor. For example, the main transistor can be subdivided into two or more sections of active regions. The gates of the main transistor cells included in a particular section are connected to the same gate electrode. As such, more than one gate electrode is provided for the main transistor and each of those gate electrodes corresponds to a different active region of the main transistor. Each section of the main transistor can be individually controlled via the corresponding gate electrode for that section. Such an arrangement effectively allows for programmable chip size on demand. As the load demand changes, so too does the effective size of the chip by individually controlling the different active regions of the main transistor.
One problem with the programmable chip size on demand approach explained above is that accurate sensing of the current flowing through the main transistor becomes more complex. A sense transistor typically is coupled in parallel with the main transistor and controlled by a single gate signal in order to mirror the current flowing through the main transistor. A current sense circuit such as an operational amplifier equalizes the voltage at a common sense node of the sense and main transistors (e.g. the source in the case of an nMOS transistors and the drain in the case of pMOS transistors), and outputs a current estimate corresponding to the current mirrored by the sense transistor. However, the on-state resistance (Ron) of the main transistor varies significantly depending on which active regions of the main transistor are on and which ones are off. As a result, the Ron of the main transistor varies as the load demand changes.
To implement a conventional sense transistor in a main transistor die having the programmable chip size on demand approach explained above requires a scaling of the current sense output of the current sense circuit as a function of which active regions of the main transistor are on and which ones are off. In other words, the ratio of the on-resistance of the sense transistor to that of the main transistor depends on which gates of the main transistor are active and which ones are not active. A conventional solution is to adjust the gain of the current sense circuit based on the operating mode of the main transistor. However, this approach requires on-the-fly scaling of the current sense output which increases the complexity and cost of the system.